720World

Little Trucks, Big World

I installed some 245/75-16 tires on Nismo rims and it of course through my speedometer off which I already expected.

Problem is, it's off by 10mph which is more than I expected and before it gets me in trouble with the local police I want to fix this problem.

 

I have "searched" before but the only threads were from back in 2008 and none came to a definite answer as to what was actually available and if it would fit or work on our trucks.

I called the local dealer and they seem to think that nothing was ever available for the 720.   Someone else suggested the use of gears from the 280z...I don't know if they will work or not.

 

Please help me if you have any experience with this issue and can help, I'd appreciate it.

 

Thanks,

Tom

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So your speedo is reading lower by 10MPH at what speed? It won't be 10 MPH at all speeds but a difference in %. For example 10 MPH at 50MPH  would be 20% . Once the % is known a cog with a suitable % increase can be fitted to raise the speedometer reading.

 

Also what was the original tire size? What year and 2wd or 4X4?

Well, according to the radar trailer on the side of road I was doing 55mph and I thought I was doing 45mph, so that will give you a baseline. LOL!!

 

The original tires were most likely 205/70 15, they were the stock steel rims that came on an '84 4x4.

I checked inside my front fender for the aluminum tag but it's not their so I could not get the rear end gear off of that unfortunately.

205 70R 15 to 215 75 R16 you will be going 55 but the speedo will actually read 47.5 (close enough) That's about 13% out. You will need a speedo cog with two less teeth on it. For example if you now have an 19 tooth you need to have a 17 tooth. fewer teeth will make it spin faster speeding up the speedometer reading.

 

Yellow.... 16

Black..... 17

Blue...... 18

White.... 18

Red....... 19


That's great info. but are these gears available for the 720?  If so, where?  The dealer was telling me that their are no speedo gear options for our trucks, and I read one person mentioning the gears from the 280ZX, but I have no idea if they are compatible with our trucks.

Also, I had stepped up to a 245, not a 215.   I know that will change the aspect ratio and I may need to go 3 teeth down.

 

Is the fluid usually high enough in the center diff. that when I pull out the cable and gear, fluid will leak out?  Just curious, so I know what I'm in for if I go to check the gear that I have currently to see which it is.

 

Thanks

Are you willing to consider marking the speedometer without changing the gears? Follow a friend at agreed upon speeds and note your speedometer readings.  

Yeah typo on the 215.

 

You would have to remove the speedo pinion and count the teeth or note the color. Then you could search for a possible fix. As I said you need about a 12% reduction in gearing to make it spin faster to read higher. The 720 came with a variety of different rear end gears and pinions to compensate the speedometer. The 720 came with 3.364, 3.545, 3.70, 3.889, 4.11, 4.375, 4.625 differentials so lots of different speedo gears to choose from. The dealer auto parts guy either said they are no longer available OR was just too lazy to help you. You figure out what pinion you have now and I'll get you a part number for one two teeth smaller. 

Ok, that'll work.

I don't know when I'll get to it though since it's not parked at my house right now, we don't have room for all the cars we have and we keep two here and two in storage.   Not the optimal choice but it works...kinda.

 

Tomorrow is supposed to be another nice day down here..around 54 or so, just depends when I get out of work.

 

"DN"  marking the speedo sound like a good idea, but I really would like it to just read correctly so I don't have to think about it everytime I check my speed and I also don't mess with the stock look of the guage.

mazspd6

If you change tire size each time you buy tires, are you going to rework the speedo each time too?

Something to consider.

 

Their isn't any need to change tire size after this, the truck is pulling great, it looks great.   I won't be going any larger that's for sure and the whole reason I changed at all was to move up to the 16 inch lightweight Nismo wheel instead.

When I bought the truck the guy had 30x9.5 15's on it and it looked great, but the wheels were rusted and needed replacement, I then went through a couple used sets trying to figure out what I wanted and decided I liked it best the way it originally was...this tire is equivalent in outside diameter and width to a30x9.5 tire.

I don't like to wake up old threads but this is my own thread and it is something I have always intended to do but just never did.    Between other issues, being down for two years during it's complete rebuild, and now having just finished installing the 71C transmission, I felt it was time to finally address the issue.

I drained the center differential and removed the speedo gear assembly and found  that I have a blue 18 tooth gear.   

It would be a safe bet that I will need a yellow 16 tooth to increase it the necessary 10 mph, or at least close to that, I need to make it read correct.

Can you use the drive gear out of any Nissan or are they different?   I searched and found a part number of  32703-78100 on Zparts.com for the yellow 16 tooth.   

To answer my own question...yes, kinda.  I compared the shaft assembly of the 16 tooth to my stock 18 tooth and it was almost completely identical.

I say "almost" because the last part of the shaft, where the slot is for the cable to be driven, is a little smaller in diameter, however, the cable fits internally exactly the way that it should it's just that the outside diameter of the 720 shaft is slightly beefier, but not by much.

I ordered a new bushings and O-ring to make it complete and will update soon with the outcome.

Below are a few pictures of the original and the 280z version.

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Another pic. from the side.

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